This invention relates generally to optical systems and more particularly to optical systems which are adapted for use in infrared missile seekers.
As is known in the art, optical systems have a wide variety of applications including use in infrared missile seekers. One type of such missile seeker includes a gimballed, scanning and focusing system, such as a catadioptric arrangement having a primary and secondary mirror, for focusing infrared energy from an external source, such as a target, into a small spot on a focal plane within the seeker. The small spot is disposed in the focal plane at a point where the optic axis of the scanning and focusing system intersects the focal plane. The secondary mirror is tilted from the scanning and focusing system's axis of rotation. As the primary and secondary mirrors rotate as a unit about the axis of rotation, the small spot, and hence the optic axis, traces, or scans, in a circle on the focal plane. The position of the center of the circle traced in the focal plane is related to the boresight error (i.e., the angular deviation of the line of sight, or boresight axis, to the target from the axis of rotation). Fixedly disposed within the focal plane is a reticle which is also gimballed within the missile's body. As the tilted secondary mirror rotates about the axis of rotation, the intensity of the infrared energy passing through the reticle is both amplitude and frequency modulated in accordance with the boresight error. Such modulated infrared energy is directed onto a large, single photodetector, fixedly mounted to the missile body, by means of a refractive collecting optical arrangement. The response of the photodetector to the modulated infrared energy impinging thereon produces an indication of the boresight error. An example of processing signals produced by a reticle to obtain angular deviation is described in U.S. Pat. No. 4,339,959 issued July 20, 1982, inventors Benjamin Klaus, Jr. and Gordon MacKenzie and assigned to the same assignee as the present invention.
As described, the scanning and focusing system is gimballed within the missile. Thus, for example, as described in U.S. Pat. No. 3,872,308, issued Mar. 18, 1975, inventors James E. Hopson and Gordon G. MacKenzie, assigned to the same assignee as the present invention, a gimbal system is coupled between the body of the missile and the scanning and focusing system to enable two degrees of freedom (i.e., pitch and yaw movement) of the scanning and focusing system within the missile. As described in U.S. Pat. No. 3,872,308, the detector is fixedly mounted to the missile. Thus, when using a reticle and a single detector, as the focusing system is gimballed in pitch and yaw, energy will be focused to arrive in focus at the reticle, and then collected at the large, single detector. The boresight error will be determined by processing the aforementioned reticle produced amplitude and frequency modulation on the energy collected by the detector.
However, recticle systems having a large, single detector may be limited in their ability to find and track targets. Further, a detector produces a noise voltage proportional to its diameter. Systems having multiple, small area detectors, such as an array of detectors, have better resolution of objects and increased sensitivity (i.e., signal-to-clutter and signal-to-noise (S/N)) ratios because of their small diameter. If the array of detectors is disposed in a detector plane fixed to the missile body, however, when the scanning and focusing system is gimballed in pitch and yaw the focal plane of the scanning and focusing system will be skewed with respect to the body fixed detector plane. Therefore, because the focal plane will be different from the detector plane an image in focus in the focal plane will not be in focus in the detector plane. In order for the image to be in focus to all the detectors in the array thereof, the plane of the detector plane would also be required to gimbal in pitch and yaw with respect to the missile body so that the focal plane and the detector plane remain in a common plane, regardless of the pitch and yaw orientation of the gimballed focusing system. However, as is further known, it is necessary to cool the detectors to cryogenic temperatures. Such cooling is typically accomplished by mounting the detectors to a Dewar flask and cryostat assembly. Thus, in a missile application having only a relatively small space for the scanning and focusing system, the array of detectors, the cryostat assembly, and the Dewar flask, it may not be possible to gimbal both the scanning and focusing system and an array of cryogenically cooled detectors in order to maintain the entire array of detectors in focus in systems requiring large gimbal angles of the scanning and focusing system.